Injectable formulations useful as contrast agents, particularly for ultrasound imaging, include suspensions of gas bubbles having a diameter of a few microns dispersed in an aqueous medium. Of particular interest are gas bubbles which are stabilized by means of suitable additives such as, for example emulsifiers, oils, thickeners or sugars, or by entrapping or encapsulating the gas or a precursor thereof in a variety of systems. These stabilized gas bubbles are generally referred to in the art as gas-filled microvesicles.
Useful gas-filled microvesicles include aqueous suspensions in which the bubbles of gas are bounded at the gas/liquid interface by a very thin envelope (film) involving a stabilizing amphiphilic material disposed at the gas to liquid interface. These suspensions are typically prepared by contacting powdered amphiphilic materials, e.g. freeze-dried preformed liposomes or freeze-dried or spray-dried lipid solutions, with air or other gas and then with an aqueous carrier, while agitating to generate a suspension of gas-filled microvesicles which can then be administered, preferably shortly after its preparation. Examples of aqueous suspension of gas-filled microvesicles and preparation thereof are disclosed, for instance, in U.S. Pat. No. 5,271,928, U.S. Pat. No. 5,445,813, U.S. Pat. No. 5,413,774, U.S. Pat. Nos. 5,556,610, 5,597,549, U.S. Pat. No. 5,827,504, WO 97/29783 and WO2004/069284, which are herein incorporated by reference in their entirety.
Among the component of lipid stabilized gas-filled microvesicles, it has also been suggested to introduce stabilizing compounds in the form of lipids bearing a hydrophilic polymer, such as phospholipids linked to a polyethyleneglycol, in variable amounts. For instance, U.S. Pat. No. 6,416,740 discloses compositions where the amount of said polymer-bearing lipid is greater than about 1 mole percent, it being at least of five mole percent when the lipid is a phospholipid bearing a polyethyleneglycol.
The Applicant has now observed that such amounts of lipids bearing a hydrophilic polymer are however not necessary to obtain stable microvesicles, as such stability can unexpectedly be obtained at very low concentrations of lipids bearing a hydrophilic polymer. The Applicant has further observed that such low amounts of lipids bearing a hydrophilic polymer may allow to entrap higher volumes of gas into the microvesicles. In addition, the Applicant has also observed that such very low concentrations of lipids bearing a hydrophilic polymer may also surprisingly result in an increased number of microvesicles.